Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes based on optical metrology are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition and other parameters of nanoscale structures. Traditionally, optical metrology measurements are performed on targets consisting of thin films and/or repeated periodic structures. These films and periodic structures typically represent the actual device geometry and material structure or an intermediate design during device fabrication.
As devices (e.g., logic and memory devices) move to ever smaller nanometer-scale dimensions and become more complex in terms of their three-dimensional geometry and selection of materials, characterization of such devices increases in difficulty. For example, high aspect ratio, three dimensional structures (e.g., some memory structures) present a particular challenge to optical metrology. Often, high aspect ratio geometry physically impedes the exposure of bottom layers to optical radiation. Hence, measurement sensitivity suffers due to low electromagnetic field intensity in the areas of interest. In another example, material opacity (e.g., increasingly used high-k materials) impedes the penetration of optical radiation to bottom layers. The lack of exposure to optical radiation dramatically reduces the measurement sensitivity. Furthermore, the lack of measurement data decreases the reliability of the decoupling of correlations among the many parameters characterizing complex structures (e.g., FinFETs). Thus, measurement of current devices with optical metrology tools (e.g., a spectroscopic ellipsometer or reflectometer) is becoming increasingly challenging.
In response to these challenges, more complex tools that acquire more signals from the target have been employed. For example, more wavelengths (e.g., deep ultraviolet and vacuum ultraviolet wavelengths), more complete information for reflected signals (e.g., measuring multiple Mueller matrix elements in addition to conventional reflectivity or ellipsometric signals), and multiple angles of illumination have been employed. In some examples, a combination of multiple optical inspection systems and non-optical inspection systems have been employed.
However, it has become clear that these approaches cannot reliably overcome fundamental challenges in sensitivity and parameter correlation for many advanced targets, especially those with complex three dimensional structures, opaque materials, or other structures with known, low parameter sensitivity or high parameter correlation. Thus, methods and systems for characterizing devices having complex three dimensional geometry and/or opaque materials at high throughput are desired.